Down syndrome is caused by an extra copy of chromosome 21.

• A. A missing chromosome 21.
• B. An extra copy of chromosome 21.
• C. A translocation involving chromosome 21.
• D. A duplication of chromosome 18.

Answer: (B)

Down syndrome most commonly results from having an extra copy of chromosome 21, so there are three copies of this chromosome in the cells (trisomy 21). This extra genetic material disrupts normal development and leads to the characteristic features and health profiles of Down syndrome. If chromosome 21 were missing, as in monosomy 21, that would usually be incompatible with life or lead to a different and much more severe set of abnormalities. A duplication of chromosome 18 would cause problems associated with trisomy 18, not Down syndrome. A translocation involving chromosome 21 can cause Down syndrome in some cases, but that involves a different mechanism and is less typical. Therefore, describing Down syndrome as an extra copy of chromosome 21 best matches its common genetic basis.